Hybrid silicon and carbon clathrates

ABSTRACT

A composition comprising a Type 1 clathrate of silicon having a Si 46  framework cage structure wherein the silicon atoms on said framework are at least partially substituted by carbon atoms, said composition represented by the formula C y Si 46-y  with 1≦y≦45. The composition of may include one or more guest atoms A within the cage structure represented by the formula A x C y Si 46-y  wherein A=H, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca. Sr, Ba, Ra, Eu, Cl, Br, or I or any metal or metalloid element and x is the number of said guest atoms within said cage structure.

FIELD OF THE INVENTION

The present invention relates to the composition and synthesis ofclathrate compounds with a silicon and carbon framework. Morespecifically, Type I clathrates of silicon and carbon are disclosed withor without guest atoms. These compounds may be suitable for use asthermoelectric materials, electronic materials, and energy storagematerials.

BACKGROUND

Silicon clathrate Si₄₆ refers to crystalline Si with a regulararrangement of 20-atom and 24-atom cages fused together through 5 atompentagonal rings (Type I clathrate). It has a simple cubic structurewith a lattice parameter of 10.335 Å and 46 Si atoms per unit cell. FIG.1 illustrates the cage structure of the Si₄₆, which belongs to the SpaceGroup Pm 3n and Space Group Number 223. The crystal structure of thesilicon clathrate (Si₄₆) is different from the common form ofcrystalline Si (c-Si), which is diamond cubic with a lattice parameterof about 5.456 Å and belongs to the Space Group Fd 3m , Number 227.

Another form of silicon clathrate is Si₃₄ (Type II clathrate) thatconsists of crystalline Si with a regular arrangement of 20 atoms and 28atom cages fused together through five-atom pentagonal rings. Type IISi₃₄ clathrate has a face-centered cubic (fcc) structure, with 34 Siatoms per fcc unit cell. The Si₃₄ clathrate has a lattice parameter of14.62 Å and belongs to the Space Group Fd 3m , Number 227. Type IIsilicon clathrate is sometimes referred to as Si₁₃₆ since the compoundcontains four fcc unit cells. A third form of silicon clathrate is amodification of the Si₄₆ type formed by removing four atoms from the24-atom cages.

SUMMARY

A composition comprising a Type 1 clathrate of silicon having a Si₄₆framework cage structure wherein the silicon atoms on the framework areat least partially substituted by carbon atoms and wherein thecomposition may be represented by the formula C_(y)Si_(46-y) with1≦y≦45. The cage structure may include guest atoms, in which case thecomposition may be represented by the formula A_(x)C_(y)Si_(46-y) with1≦y≦45 and wherein A is a guest atom in the cage structure and x is thenumber of guest atoms in the cage wherein x has a value such that thecage structure undergoes a volume expansion of less than or equal to50.0%.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description below may be better understood with referenceto the accompanying figures which are provided for illustrative purposesand are not to be considered as limiting any aspect of the invention.

FIG. 1 illustrates Si₂₀ and Si₂₄ cages as the building of therepresentative Si₄₆ clathrate.

FIG. 2 illustrates a Type I C_(y)Si_(46-y) clathrate where y is greaterthan or equal to 1 and less than or equal to 45.

FIG. 3 is a structural representation of a possible Type IA_(x)C_(y)Si_(46-y) clathrate compound.

FIG. 4 illustrates the computed values of the energy change of formation(Delta E) per atom for the identified structures.

FIG. 5 provides the computed values of energy of formation for a numberof Type I A_(x)C_(y)Si_(46-y) clathrates as compared against those ofC₄₆, Si₄₆, and Ba₈Si₄₆.

FIG. 6 shows powder XRD data of the indicated arc-melted productcontaining Ba₆C₂₀Si₂₆.

FIG. 7 shows powder XRD measured for Ba₈C₂₀Si₂₆ minus those of SiC andSi compared with those of Ba and the theoretical spectra of Type Iclathrate structure of Ba₈C₆Si₄₀, Ba₈C₂₀Si₂₆, and Ba₈C₂₃Si₂₃ as well asthose of Ba.

FIG. 8 shows powder XRD measured for ball milled Ba₈C₂₀Si₂₆ afterarc-melting of powdered admixture of Ba, SiC and Si.

DETAILED DESCRIPTION

Theoretical computations have shown that both Type I carbon clathrate(C₄₆) and Type II carbon clathrate (C₁₃₆ or C₃₄) may exist as metastablephases under high pressures. Insertion of guest atoms such as Li, Na, orBa into the cage structures may also be feasible under high pressures.However, the energy of formation for the Type I and Type II carbonclathrates are extremely high and syntheses of neither Type I nor TypeII carbon clathrates have been reported.

Computational studies were therefore initially undertaken on the Type Iclathrate allotropes of silicon which revealed that the silicon atoms onthe Si₄₆ framework of the cage structure can be partially substituted bycarbon atoms to form a hybrid silicon-carbon clathrate, which can berepresented by the chemical formula C_(y)Si_(46-y). FIG. 2 shows arepresentation of the Type I C_(y)Si_(46-y) clathrates, wherein y isgreater than or equal to 1 and less than or equal to 45 (1≦y≦45). Forexample, y may have a value of 4-40. The arrows in FIG. 2 identifypossible placement of the four identified carbon atoms.

Furthermore, guest atoms can be inserted into the cage structure tostabilize the hybrid silicon carbon clathrate by reducing the energy offormation n to form a class of new hybrid silicon and carbon clathrates,represented as A_(x)C_(y)Si_(46-y). In this situation y is again greaterthan equal to 1 and less than or equal to 45 (1≦y≦45) and A=H, Li, Na,K, Rb, Cs, Fr, Be, Mg, Ca. Sr, Ba, Ra, Eu, Cl Br and I and any metal ormetalloid element capable of occupying the empty spaces inside the cageof the Type 1 clathrate structure. Metals may therefore include any oneof alkali metals, alkaline earth metals, transition metals,post-transition metals or lathanoids. Metalloids may include B, Si, Ge,As, Sb or Te.

The value of x in the above formula A_(x)C_(y)Si_(46-y) corresponds tothe number of guest atoms A residing in the cage. The number of guestatoms, x, that can occupy inside the cage structure will depend on theatomic size of A. For relatively large atoms such as Ba, which has anatomic radius of 222 picometers, the number of Ba atoms intercalatedwithin the cage structure is preferably eight or less (0≦x≦8). Forrelatively small atoms such as Li, which has an atomic radius of 152picometers, the number of Li atoms, x, intercalated within the cagestructures will depend on the specific form and stoichiometric ratio ofC and Si. Ultimately the value of x for any guest atom “A” is limited toa value defined by the onset of significant expansion of the latticeparameter, beyond which irreversible structural damage occurs in thebulk material. It should be noted that while one may determine atheoretical capacity of guest atoms before one would expect expansion(e.g., in the case of Ba a theoretical capacity of around 8 and in thecase of Li a theoretical capacity of around 23), it has been observedthat one may exceed the theoretical capacity depending upon the observedelasticity of the cage (C_(y)Si_(46-y)). Accordingly, irreversiblestructural damage may occur beyond the theoretical capacity of any givenguest atom and may be determined experimentally.

It may be appreciated therefore that the clathrate structure defined bythe equation A_(x)C_(y)Si_(46-y) may be understood as one that, uponintercalcation of a guest atom A, the value of x is selected such thatthe cage structure will preferably undergo a volume expansion of lessthan or equal to 50.0%, or in the range of 0.1% to 50.0% at 0.1%increments. In related context, the C—Si clathrate structures herein aresuch that upon deintercalcation, preferably undergo a volume change(contraction) of 50.0% or less, or in the range of 0.1% to 50.0% at 0.1%increments.

FIG. 3 shows a structural representation of a possible Type IA_(x)C_(y)Si_(46-y) clathrate compound with four carbon atoms and thesingle guest atom “A” as indicated. The computed values of the energychange of formation per atom for representative Ba₈C_(y)Si_(46-y), C₄₆,C₄₀Si₆, and C₂₃Si₂₃ clathrates are compared with those for Si₄₆ andBa₈Si₄₆ in FIG. 4. The positive values for the energy of formation atthe minima of the energy change curves indicate that theseBa₈C_(y)Si_(46-y) clathrate compounds are metastable.

FIG. 5 provides the computed values of the energy of formation for anumber of Type I A_(x)C_(y)Si_(46-y) clathrates as compared againstthose of C₄₆, Si₄₆, and Ba₈Si₄₆. The comparison shows that value of theenergy of formation for C_(y)Si_(46-y) and A_(x)C_(y)Si_(46-y) ispositive and it generally increases with decreasing values of thelattice parameter.

Accordingly, the hybrid silicon and carbon clathrate disclosed herein ispreferably obtained by substituting some silicon atoms on the Type Isilicon clathate framework, Si₄₆, with carbon atoms to produce asilicon-carbon framework that is represented by C_(y)Si_(46-y) andconsists of y carbon atoms and 46-y silicon atoms with a regulararrangement of 20-atom and 24-atom cages fused together through 5 atompentagonal rings (Type I clathrate). It has a simple cubic structurewith a lattice parameter in the range of 6.72 Å to 11.20 Å and acombined sum of 46 Si and C atoms per unit cell. Like Si₄₆, the crystalstructure of the C_(y)Si_(46-y) clathrate belongs to the Space Group Pm3n, Number 223.

The hybrid silicon carbon clathrate allotropes described herein may bepreferably prepared via an arc melt technique, which has been employedin connection with the preparation of metallic and refractory alloys.Such preparation may be represented by the following:

${x\; {Ba}} = {{y\; {SiC}} + {\left( {46 - {2y}} \right){{Si}\overset{\Delta}{}{Ba}_{x}}C_{y}{Si}_{46 - y}}}$

In the above equation, Δ denotes the energy (heat and electrical)delivered to the powdered admixture, and x=1-8 and y=6-23.

Representative examples are identified below. In either case, theintroduction or replacement of guest atoms, for example replacement ofBa with Li atoms, may be afforded through the methods described in U.S.application Ser. No. 13/109,704, whose teachings are incorporated byreference. Representative examples therefore include, but are notlimited to, Ba_(x)C₆Si₄₀, Ba_(x)C₁₆Si₃₀, Ba_(x)C₂₀Si₂₆, Ba_(x)C₂₃Si₂₃,Li_(x)C₆Si₄₀, Li_(x)C₁₆Si₃₀, Li_(x)C₂₀Si₂₆, Li_(x)C₂₃Si₂₃, or similarpermutations of C and Si.

Accordingly, in the present invention, Ba₈C₂₀Si₂₆ has been synthesizedby arc-melting appropriate amounts of Ba, SiC, and Si as the startingmaterials. Admixtures of Ba, Si, and SiC (in the proportion of 106.2 gBa, 16.29 g of Si, and 77.52 g of SiC powders) was arc-melted to provide200 grams of product, containing Ba₆C₂₀Si₂₆ plus some amounts ofunreacted starting materials. Powder x-ray diffraction (XRD) data of thearc-melted product (i.e., not purified) containing Ba₆C₂₀Si₂₆ ispresented in FIG. 6. Some of the reflection peaks in the XRD spectra, asillustrated, correspond to unreacted Ba, SiC, and Si starting materials,as well as some BaSi₂. However, the remaining reflections in the XRDspectra do not belong to Ba, Si, BaSi₂, and Si, and have been assignedto the Ba₆C₂₀Si₂₆ clathrate structure of the present invention. Theseassignments are corroborated by additional analysis for compoundidentification, which follows.

Further analysis of the XRD results was carried out by subtracting theSiC and Si reflection peaks from the Ba₈C₂₀Si₂₆ spectra. The remainingreflection peaks were then compared with the theoretical-computed XRDspectra for Type I clathrate structure of Ba₈C₆Si₄₀, Ba₈C₂₀Si₂₆, andBa₈C₂₃Si₂₃ as well as those of Ba. These comparisons are presented inFIG. 7. The theoretically-computed reflections for Ba₈C₆Si₄₀, Ba₈C₂₀Si₂₆and Ba₈C₂₃Si₂₃ were obtained by first optimizing the Type I clathratestructures using first principles Car-Parrinello Molecular Dynamics(CPMD) computations to derive the equilibrium crystallographicparameters, followed by computing the corresponding reflections in theXRD spectrum using the analysis and visualization software calledDiamond. The comparison indicates that a Type I clathrate compound ispresent in the arc-melted Ba₈C₂₀Si₂₆ product. The crystal structure ofthis clathrate compound is close to those of Ba₈C₆Si₄₀, Ba₈C₂₀Si₂₆, andBa₈C₂₃Si₂₃ based on the characteristic reflections at 2θ of 18°, 21°,30°, and 32° for the silicon carbon clathrates.

Some of the as-synthesized Ba₈C₂₀Si₂₆ materials were ball-milled intofiner powders and subsequently characterized by powder XRD. FIG. 8presents the XRD spectrum observed in the ball-milled materials, whichindicates the presence of Ba, BaSi₂, SiC, and Si in the reflectionpatterns for the ball-milled Ba₈C₂₀Si₂₆ powders. The characteristicreflections for Type I clathrate at 2θ of 18°, 21°, 30°, and 32° haveall disappeared and have been replaced by those corresponding to BaSi₂in the ball-milled materials. The results indicate that the Type Iclathrate material in the as-synthesized Ba₈C₂₀Si₂₆ is metastable, andit can be made to transform to the lower energy compound BaSi₂ by theaction of ball-milling. This finding is consistent with thefirst-principles computations shown in FIG. 4 and FIG. 5, which showthat Ba₈C₆Si₄₀, Ba₈C₂₀Si₂₆, and Ba₈C₂₃Si₂₃ are metastable compounds withpositive energies of formation. Among the three compounds, Ba₈C₆Si₄₀ hasthe lowest relative energy of formation, followed by Ba₈C₂₀Si₂₆, andBa₈C₂₃Si₂₃. It is therefore contemplated herein that the finding thatBa₈C₂₀Si₂₆ can be synthesized by arc-melting using SiC as the startingmaterial suggests that the technique may be employed for the synthesisof other hybrid silicon and carbon clathrates, such as Ba₈C₆Si₄₀ andBa₈C₂₃Si₂₃.

In accordance with all of the above, the present invention identifiesbeneficial attributes and utility for Type I clathrates of silicon andcarbon with or without guest atoms. In such compositions, the bandstructure and, in particular, the electrochemical work function of thealloy clathrates may be tuned by either altering the number of carbonatoms on the hybrid silicon and carbon framework or by altering theguest atoms inserted into the cage structure of clathrate system. Theseelectronic characteristics make this class of Type I clathrates suitablefor applications as thermoelectric, electronic, and energy storagematerials.

The identified hybrid carbon and Si framework may lead to delocalizationof the band structure, reduce the band gap and increase the electronicconductivity of the clathrate compound. The presence of carbon atoms onthe clathrate framework can result in a relatively smaller latticeconstant and relatively less empty space in the cage structure so thatthere is more electronic interaction between the guest atom and the Siand C atoms on the surrounding framework. It is contemplated that theseinteractions can be tuned to enhance the Seebeck effects and electronicconductivity, alter the band gap, and reduce the thermal conductivity byadjusting the number of carbon atoms on the framework, as well as thesize and type of guest atoms inside the cage structure. For applicationsas energy storage materials, the band structure and, in particular, theelectrochemical work function of the anode and cathode for combinationsof electrodes with unique clathrate-alloy compositions may be tuned tobe compatible with the rest of the battery system, including theabsolute energies of the highest occupied molecular orbital (HOMO) andthe lowest unoccupied molecular orbital (LUMO) of the electrolyte.

This tunability of the anode and cathode may be accomplished byadjusting the stoichiometric ratios of Si/C such that a desirableopen-circuit potential is obtained in the charged state of the batterywithin a thermodynamically-stable energy range of the electrolyte. Thus,using appropriate ratios and/or elemental forms of Si/C framework atomsmay yield a small work function necessary for the clathrate-alloycomposition to function as an anode, whereas different ratios and/orelemental form of Si/C framework atoms may be used to yield a large workfunction necessary for the silicon-carbon clathrate-alloy composition tofunction as a cathode. The battery couple (anode+cathode) that resultsis therefore based on a single class of material chemistry, though withunique ratios and elemental forms of Si/C framework atoms.

Accordingly, it may be appreciated that the Type 1 silicon-carbonclathrate alloy herein of the formula C_(y)Si_(46-y) with 1≦y≦45, or thesilicon-clathrate alloy herein of the formula A_(x)C_(y)Si_(46-y) with1≦y≦45 and A comprising A=H, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca. Sr, Ba,Ra, Eu, Cl Br and I and any metal or metalloid element capable ofoccupying the empty spaces inside the cage of the Type 1 clathratestructure, may be configured such that they have one or more of thefollowing characteristics: (1) be of particle form having a largestlinear dimension of 0.1 μm to 100.0 μm; (2) be of electrode form whereinthe electrode comprises a metal substrate and the clathrate alloystructure is present on the surface of the metal substrate; (3) be ofthe formula A_(x)C_(y)Si_(46-y) wherein A is Li; (4) be of anodeelectrode form in a Li battery; (5) be of cathode electrode form in a Libattery. Reference to a Li-battery may be understood as a rechargeablebattery in which lithium ions move from a negative electrode to apositive electrode during discharge and back when charging. Duringdischarge lithium ions Li+ carry current from the negative to thepositive electrode through a non-aqueous electrolyte and separatordiaphragm.

1. A composition comprising a Type 1 clathrate of silicon having a Si₄₆framework cage structure wherein the silicon atoms on said framework areat least partially substituted by carbon atoms, said compositionrepresented by the formula C_(y)Si_(46-y) with 1≦y≦45.
 2. Thecomposition of claim 1 wherein y has a value of 4-40.
 3. The compositionof claim 1 wherein said composition is in the form of a batteryelectrode.
 4. The composition of claim 3 wherein said electrodecomprises an anode electrode in a Li battery.
 5. The composition ofclaim 3 wherein said electrode comprises a cathode electrode in a Libattery.
 6. The composition of claim 1 further including one or moreguest atoms A within said cage structure, represented by the formulaA_(x)C_(y)Si_(46-y) wherein A=H, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca. Sr,Ba, Ra, Eu, Cl, Br, or I or any metal or metalloid element and x is thenumber of said guest atoms within said cage structure.
 7. Thecomposition of claim 6 wherein x is greater than 0 and less than orequal to
 8. 8. The composition of claim 6 where A is Li.
 9. Thecomposition of claim 6 wherein x has a value such that the cagestructure undergoes a volume expansion of less than or equal to 50.0%.10. A composition comprising a Type 1 clathrate of silicon having a Si₄₆framework cage structure wherein the silicon atoms on said framework areat least partially substituted by carbon atoms, said compositionrepresented by the formula A_(x)C_(y)Si_(46-y) with 1≦y≦45 and wherein Ais a guest atom in said cage structure and x is the number of guestatoms in said cage structure where A=H, Li, Na, K, Rb, Cs, Fr, Be, Mg,Ca, Sr, Ba, Ra, Eu, Cl, Br, or I or any metal or metalloid element. 11.The composition of claim 10 wherein x has a value such that said cagestructure undergoes a volume expansion of less than or equal to 50.0%.12. The composition of claim 10 where A is Li.
 13. The composition ofclaim 10 wherein y has a value of 4-40.
 14. The composition of claim 10wherein said composition is in the form of a battery electrode.
 15. Thecomposition of claim 14 wherein said electrode comprises an anodeelectrode in a Li battery.
 16. The composition of claim 14 wherein saidelectrode comprises a cathode electrode in a Li battery.